Intervention : In part A of the imaging trial, a dose finding imaging study will be performed to assess the optimal tracer protein dose of 89Zr-pembrolizumab and the optimal interval between tracer injection and scanning. Approximately 3 cohorts of about 2-3 patients each will undergo 89Zr-pembrolizumab-PET imaging before start of treatment with pembrolizumab. In part B, 12 eligible patients will undergo 89Zr-pembrolizumab-PET imaging at baseline, with the optimal tracer protein dose and scanning schedule as determined in part A. In total, a maximum of 21 patients will be included.The purpose of part B of the study is to analyze the whole body distribution and pharmacokinetics (PK) of 89Zr-Pembrolizumab. Tumor and normal organ radioactive tracer uptake will be quantified as standardized uptake values (SUV). Tumor biopsies will be collected at baseline and studied for various characteristics including Programmed cell death-ligand 1 (PDL-1), PD-1 expression and tumor infiltrating lymphocytes. Pembrolizumab treatment will be administered as standard of care Main study parameters/endpoints: To evaluate whole body distribution of 89Zr-pembrolizumab in patients with locally advanced or metastatic melanoma.